In internal combustion engine,atmospheric air is essentially required to burn the fuel.The Oxygen in air,helps for proper burning of fuel.To obtain excellent combustion result from fuel,the air and the fuel must be mixed in proper ratio.For complete combustion,the air-fuel ratio is approximately 15 : 1 by weight.This ratio is known as Chemically correct air-fuel ratio.This ideal ration in any internal combustion engine.
However,the air-fuel ratio can be range from 20 : 1 to 8 : 1,in this range also combustion of fuel can occur.Any ratio outside of this range is either too rich or too lean to sustain flame propagation.
As per requirement of engine,the carburettor provide an air-fuel ratio,which must be within combustion range.Engine is cold at the time of starting so,very rich mixture is required.Rich mixture is also required at time of idling and producing maximum power.During the normal running,a comparatively lean mixture can be used.
For petrol engine,different air-fuel ratios are required under various conditions of load.These are as discussed below.
01)Air-Fuel Ratio For Starting:-
Very rich mixture ( 10 : 1 ) is required at starting of engine.During starting very small amount of fuel is vaporises and rest of it stay in the liquid state so as to give an ignitable mixture.
02)Air-Fuel Ratio For Idling:-
An idling,engine demands a rich mixture,which can be made leaner as the throttle is gradually opened.During idling,the pressure in the inlet manifold is about 20 to 25% of atmospheric pressure.At suction stroke,inlet valve opens and the product of combustion trapped in the clearance volume,expands in the inlet manifold.Latter when the piston moves downwards,the gases along with the fresh charges go into the cylinder.A rich mixture must be supplied during idling,to counteract the tendency of dilution and to get an ignitable mixture.
03)Air-Fuel Ratio For Medium Load:-
Most of the time,engine is running in medium load condition,therefore,it is desirable that the running should be most economical in this condition.So a lean mixture can be supplied,as engine has low fuel consumption at medium load.For multi cylinder engine,slightly more fuel is required due to maldistribution of fuel.
04)Air-Fuel Ratio For Maximum Power Range:-
When maximum power is required,the engine must be supplied with rich mixture as the economy is of no consideration.As the engine enters in the power range,the spark must be retarded otherwise knocking would occur.A lean mixture burns at latter part of working stroke.As the exhaust valve expose to high temperature gases and have very less time to cool down.Moreover,the excess air in the lean mixture may cause an oxidising action on the hot exhaust valve and leads to failure.
05)Air-Fuel Ratio For Acceleration:-
Even during normal running,sometimes more power is required for a short period such as to accelerate the vehicle for overtaking etc.During this period rich mixture is required.
I hope,this will help you to understand something about combustion of fuel at different running conditions of the engine.
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